1. Field of the Invention
This invention relates to a method for determining a desired image signal range in an image signal representing a radiation image, which image signal range represents information recorded within a desired image region to be viewed in the radiation image
2. Description of the Prior Art
Techniques for reading out a recorded radiation image in order to obtain an image signal, carrying out appropriate image processing on the image signal, and then reproducing a visible image by use of the processed image signal have heretofore been known in various fields. For example, as disclosed in Japanese Patent Publication No. 61(1986)-5193, an X-ray image is recorded on an X-ray film having a small gamma value chosen according to the type of image processing to be carried out, the X-ray image is read out from the X-ray film and converted into an electric signal, and the electric signal (image signal) is processed and then used for reproducing the X-ray image as a visible image on a copy photograph or the like. In this manner, a visible image having good image quality with high contrast, high sharpness, high graininess or the like can be reproduced.
Also, when certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored during its exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor. As disclosed in Japanese Unexamined Patent Publication Nos 55(1980)-12429, 56(1981)-11395, 55(1980)-163472, 56(1981)-104645, and 55(1980)-116340, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object such as the human body in order to store a radiation image of the object thereon, and is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored during exposure to the radiation. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted into an electric image signal, and the image signal is used to reproduce the radiation image of the object as a visible image on a recording material such as photographic film, on a display device such as a cathode ray tube (CRT), or the like.
Radiation image recording and reproducing systems which use stimulable phosphor sheets are advantageous over conventional radiography using silver halide photographic materials in that images can be recorded even when the energy intensity of the radiation to which the stimulable phosphor sheet is exposed varies over a wide range. More specifically, since the amount of light which the stimulable phosphor sheet emits upon stimulation varies over a wide range and is proportional to the amount of energy stored thereon during its exposure to the radiation, it is possible to obtain an image having a desirable density regardless of the energy intensity of the radiation to which the stimulable phosphor sheet was exposed. In order to obtain a desirable image density, an appropriate read-out gain is set when the emitted light is being detected with a photoelectric read-out means and converted into an electric signal to be used in the reproduction of a visible image on a recording material, such as photographic film, or on a display device, such as a CRT.
In order for an image signal to be detected accurately, certain factors which affect the image signal must be set in accordance with the dose of radiation delivered to the stimulable phosphor sheet and the like. A novel radiation image recording and reproducing system which accurately detects an image signal has been proposed. The proposed radiation image recording and reproducing system is constituted such that a preliminary read-out operation (hereinafter simply referred to as the "preliminary readout") is carried out in order approximately to ascertain the radiation image stored on the stimulable phosphor sheet In the preliminary readout, the stimulable phosphor sheet is scanned with a light beam having a comparatively low energy level, and a preliminary read-out image signal obtained during the preliminary readout is analyzed. Thereafter, a final read-out operation (hereinafter simply referred to as the "final readout") is carried out to obtain the image signal, which is to be used during the reproduction of a visible image. In the final readout, the stimulable phosphor sheet is scanned with a light beam having a high energy level, and the radiation image is read out with the factors affecting the image signal adjusted to appropriate values on the basis of the results of an analysis of the preliminary read-out image signal.
The term "read-out conditions" as used hereinafter means a group of various factors, which are adjustable and which affect the relationship between the amount of light emitted by the stimulable phosphor sheet during image readout and the output of a read-out means. For example, the term "read-out conditions" may refer to a read-out gain and a scale factor, which define the relationship between the input to the read-out means and the output therefrom, or to the power of the stimulating rays used when the radiation image is read out.
The term "energy level of a light beam" as used herein means the level of energy of the light beam to which the stimulable phosphor sheet is exposed per unit area. In cases where the energy of the light emitted by the stimulable phosphor sheet depends on the wavelength of the irradiated light beam, i.e. the sensitivity of the stimulable phosphor sheet to the irradiated light beam depends upon the wavelength of the irradiated light beam, the term "energy level of a light beam" means the weighted energy level which is calculated by weighting the energy level of the light beam, to which the stimulable phosphor sheet is exposed per unit area, with the sensitivity of the stimulable phosphor sheet to the wavelength. In order to change the energy level of a light beam, light beams of different wavelengths may be used, the intensity of the light beam produced by a laser beam source or the like may be changed, or the intensity of the light beam may be changed by moving an ND filter or the like into and out of the optical path of the light beam. Alternatively, the diameter of the light beam may be changed in order to alter the scanning density, or the speed at which the stimulable phosphor sheet is scanned with the light beam may be changed.
Regardless of whether the preliminary readout is or is not carried out, it has also been proposed to analyze the image signal (including the preliminary readout image signal) obtained and to adjust the image processing conditions, which are to be used when the image signal is processed, on the basis of the results of an analysis of the image signal. The proposed method is applicable to cases where an image signal is obtained from a radiation image recorded on a recording medium such as conventional X-ray film, as well as to systems using stimulable phosphor sheets.
When the radiation image, which has been recorded on a recording medium, is reproduced as a visible image on a photographic material or the like, only certain portions of the total radiation image will be used. In order for the read-out conditions for the final readout and/or the image processing conditions to be set correctly, it is necessary to consider which portions of the radiation image are to be used and therefore are required to have an appropriate image density in the reproduced image.
FIG. 10 is a schematic view showing an example of the radiation image of the cervical region of a human body.
By way of example, for radiation images of the cervical region, only the image information concerning the cervical vertebra image A and the surrounding soft tissues is usually necessary (for a diagnosis of an illness to be made). The image information from other parts of the radiation image, i.e. a background region B upon which radiation impinged directly during the recording of the radiation image on the recording medium, a jaw image C, and a shoulder image D, is not necessary. (The amount of radiation to which the recording medium was exposed is largest in the background region B. Also, herein, the level of the image signal will increase as the amount of radiation to which the recording medium is exposed becomes larger ) In such cases, it is desired that the read-out conditions for the final readout and/or the image processing conditions be set such that the cervical vertebra image A and the image of the surrounding soft tissues are reproduced with an appropriate image density.
In order to satisfy such a requirement, the applicant has proposed a novel method in, for example, Japanese Unexamined Patent Publication No. 60(1985)-156055.
The proposed method comprises the steps of: determining the histogram of the preliminary read-out image signal obtained during the preliminary readout, calculating the maximum image signal level Smax and the minimum image signal level Smin of the desired image signal range from the histogram, and adjusting the read-out conditions for the final readout so that the maximum image signal level Smax and the minimum image signal level Smin correspond respectively to the maximum signal level Qmax and the minimum signal level Qmin which are determined by the maximum density Dmax and the minimum density Dmin of the correct density range in the reproduced visible image. The proposed method is applicable also when the image processing conditions are adjusted in a system wherein no preliminary readout is carried out, or the like.
In the course of carrying out the proposed method, it is necessary to calculate Smax and Smin accurately. However, for example, for images of the cervical region, the levels of the image signal components representing the jaw image C and the shoulder image D are lower than the levels of the image signal components representing the desired image region, i.e. the cervical vertebra image A and the image of the surrounding soft tissues. In such cases, Smax and Smin cannot be found sufficiently accurately with the method disclosed in Japanese Unexamined Patent Publication No. 60(1985)-156055 wherein Smax and Smin are calculated using a frequency threshold value which has been determined from the histogram of the image signal representing the whole radiation image with reference to what portion of an object is recorded and the image recording method used.
FIG. 11 is a graph showing the probability density function of the image signal representing the whole radiation image of the cervical region shown in FIG. 10.
By way of example, if the probability density function of the image signal representing the desired image region is composed of a section I corresponding to the jaw image and the shoulder image, a section II corresponding to the cervical vertebra image, and a section III corresponding to the image of soft tissues such as a skin and the probability density function of the image signal representing the desired image region extends approximately over the probability density function of the whole image signal except for a section IV which corresponds to the background region and which can be distinctly discriminated from the other sections because of its shape, then no section corresponding to an undesired image region is present on the side of the probability density function corresponding to the lower values of the image signal (the lower values of the image density). In such cases, Smax and Smin of the desired image signal range representing the desired image region can be calculated approximately accurately with a frequency threshold value which is determined from the shape of the probability density function of the image signal representing the whole radiation image. However, in cases where the part of the probability density function corresponding to the desired image region is composed only of a section such as section II and a section such as section III, and the section I corresponding to the jaw image and the shoulder image falls in the undesired image region and is adjacent to the sections corresponding to the desired image region at the lower image signal value end of the density function, it is not always possible for Smax and Smin of the desired image signal range representing the desired image region to be calculated accurately with the method wherein Smax and Smin are calculated from the probability density function of the whole image signal using merely a predetermined frequency threshold value.
One of methods for solving the aforesaid problems has been proposed by the applicant in Japanese Patent Application No. 62(1987)-207212. The proposed method is premised on the assumption that a radiation image recorded on a recording medium is composed of an object image, and two background regions which are separated by the object image. With the proposed method, an image signal representing the radiation image is converted with a predetermined threshold value into a two-valued system in order to form a binary image which comprises a region approximately corresponding to the object image and two areas approximately corresponding to the background region. Thereafter, the threshold value is sequentially made smaller, and the formation of the binary image is repeated until the two areas approximately corresponding to the background region become connected. A desired image signal range is determined from the image signal components which correspond to the picture elements associated with the two areas which form the background region and have become connected.
The aforesaid method is premised on the assumption that the background region in the radiation image is formed from two areas which are separated by the object image as shown in FIG. 10. Many radiation images of, for example, the cervical region have two areas forming the background region as shown in FIG. 10. However, some of the radiation images of the cervical region have only a single area forming the background region as shown in FIG. 1. In such cases, the aforesaid method cannot be applied. Therefore, there is the risk that the read-out conditions for the final readout and/or the image processing conditions will not be adjusted appropriately and that the reproduced visible image will have poor image quality and be unsuitable for, in particular, diagnostic purposes.